Electrostatic electron lens system



Jan. 23, 1951 B. J. MAYO ELECTROSTATIC ELECTRON LENS SYSTEM Filed Dec.20, 1949 l hllillr llnvllltln IVIIIIII l l l I ll [five/7751' BERNARDJOSEPH MAYO r P Patented Jan. 23, 1951 UNITED STATES PATENT OFlCEELECTROSTATIC ELECTRON LENS SYSTEM Bernard Joseph Mayo, Hayes, England,assignor to Electric & Musical Industries Limited, Hayes, England, acompany of Great Britain Application December 20, 1949, Serial No.134,005 In Great Britain December 10, 1948 4 Ciai'ms.

The invention relates to electrostatic electron lens systems forfacuss'm' composite beams of electrons as distinct from lenses forrocussin fasciculated beams. I

Electrostatic electron lenses for focussing composite beams, such as anelectron image of an optical object, onto a mosaic target'ele'ctro'de orfluorescent screen are known and have comprised two cylindricalelectrodes to which different potentials are applied for setting up anelectrostatic focussing held. The focussing field may serve to focus anelectron image emitted from a photo-electric cathode, which is disposedadjacent to the electrode maintained at the lower potential, but it isfound with these lenses that considerable curvature of the 'focus'sedelectron image occurs. Thus, assuming that the photoelectric cathode isplane, it is of course desirable that a plane image be formed on thetarget electrode or fluorescent-screen. However, with the known form oflens referred to above, the electron ima'g'e, instead-"of being focussedina plane, is in focus on-an-acutely curved surface which is concavefacing therathode withthe result that, if it is desired to employ aplane target electrodeor fluorescent -screen, the image, whilst being infocus on the axis'of the system, is-nevertheless widely out of *focus atpositions remote from the axis. In this known type of lens the radius ofcurvature "of the image with -a magniflcationfacto'r of -lL7 is found tobe as much as0.13 of the "radius of the second cylindrical electrode ofthe lens.

The object-of the present invention-is to .provide an improvedelectrostatic-electron lens system which does not introduceas much imagecurvature as in the=known type of-lens referred to above.

tained at a potential which is less than (and preferably less than 0.4of the'potentialapplied to an electrode of saidsecond lens-which isnearer to said source.

Lens systems in.-accordanc'e with the present invention can bedesignedtoproduce unity magnification or magnification greater thanunity or demagnification.

In order that the said invention may be clearly understood and readilycarried into effect it will now be more fully described with referenceto the accompanying drawings, in which:

Figure 1 represents diagrammatically the known form of lens referred toabove, and,

Figure 2 is a similar view illustrating a lens according to oneembodiment of the present in vention.

As shown in Figure 1 of the drawings'the lens comprises a air of tubularelectrodes I and 2 which are of circular form'in cross-section, theelectrode I being connected to a photoelectric cathode 3 which emitselectrons when anoptical image is projected thereon, the lens formed bythe electrodes I and '2 being intended to focus the electron image on toa plane surface 4 which may be a target electrode or a fluorescentscreen. In operating the lens, the electrode I may be maintained at thepotential of the cathode '3, which may be zero potential, and theelectrode -2 is maintained at a'positive potential. The maintenance ofthe two electrodes I and 2 at the different potentials mentionedserves'to set up an electrostatic 'fOCuSSiIlg field, the equi p'otentialsurfaces of which are indicated by the full lines 5. The dotted linesindicated in Figure l illusstrate pencils of electrons emitted from thephoto-electric cathode, and it will be observed from this figure thatthe electron pencils are rather wide and, furthermore the image pointsof the axial, zonal and marginal electron pencils indicated ate, Iand-'8 respectively show that the curvature'in the image isconsiderable, so that, whilst the axial e'lectron pencils will'be infocus at the surface --4, the marginal electron pencils will be widelyout of focus. In this form of lens the curvature of the image formed onthe surface 4 is as high-as'0.l3 of'the radius of the electrode '2.

Figure 2 of the drawings illustrates a lens in accordance with thepresent invention and in this figure the photoelectric'cathodeisvindicated by the reference numeral 9 and the plane surface'such asthe target electrode or fluorescent screen on which the iinage is to be.focussed is indicated at 4. Adjacent to the cathode 9 is a series oftubular or ring-shaped electrodes I0, II and I2 followed by a longertubular electrode I3 which together with the electrodes I0, II and I2form a'flrst'electron lens. The electrode I3 is-follo'wed'by 'afurthertubular electrode I4 the electrodes I3'-an'd I4 forming a secondelectron lens. In'one specific example of a lens system according -'tothe invention, the photo-electric cathode'll may have a diameter of 5centimetres, the electrode Ii! which is connected to the cathode 9 adiameter of 5 centimetres and a length of 0.4 centimetre, the electrodesII and I2 a diameter of 5 centimetres and each a length of 0.5centimetre, the electrode It a diameter of 5 centimetres and a length of8 centimetres and the electrode I4 a diameter of 5 centimetres and alength of 17 centimetres. In operation the electrodes II], II and I2 maybe maintained at the same potential as the photo-electric cathode 9namely at zero potential, the electrode l3 at a positive potential of5000 volts and the electrode Id at a positive potential which is lessthan 0.4 of the potential of the electrode I3. Preferably the potentialof the electrode I4 is between 500 and 1000 volts, say at 600 volts. Theequipotential surfaces set up by the first electron lens are indicatedin full lines at 15 and the pencils of electrons emitted by thephoto-electric cathode 9 by the dotted line Hi. The equi-potentialsurfaces set up between the electrodes l3 and I l are indicated in fulllines 11. It will be observed from a comparison of Figures 1 and 2 thatin the arrangement according to the invention, the pencils of electrons16 are narrow 'owing to the relatively higher field which is produced atthe cathode 9 with the result that all of the rays of the penciltraverse a similar field :and the depth of focus is small and thepencils are converged towards the axis of the system. "The electrons insaid pencils are initially slightly (converged by the first electronlens and they then 'move in a nearly field-free region before theelectrons in said pencils are converged and focussed by the second lensformed between the electrodes 13 and I l. Hence, by the time theelectron pencils reach this last-mentioned lens they are near the axisof the system, so that this latter lens introduces little aberration.The result of the lens system according to the invention is that thecurvature of the image formed on the surface 4 is less compared with thelens shown in Figure 1 as is evidenced from the image points of themarginal electron pencils indicated at H! in Figure 2. With a lenssystem according to the invention with the dimensions of electrodes andpotentials as stated above the radius of curvature of the images wasfound to be 0.3 of the diameter oi t1-h7e electrode M with amagnification factor Although, a stated above, the electrodes l0, H andI2 may be maintained at the same potential as the photo-electric cathode9, it may, in some cases, be advantageous to maintain the electrodes Hand I2 at progressively higher potentials than that of the cathode 9.The size of the various electrodes of the system and the potentialsapplied thereto control the size of the image formed on the surface 4and obviously the size of these electrodes and the potentials will beadjusted to produce a desired size of image.

In optics the well known Petzvals formula can be used to determine thecurvature of an image and such formula can also be used inelectronoptics. Such formula is:

1 N l i' i i (1) where C is the radius of curvature of the image surfaceat the axis, Nn' the refractive index of the image space, and N N arethe indices on either side of the jth refracting surface of radius r Forelectron optics, since the refractive index is proportional to Vii, wehave where V is the potential of the equipotential g. If the surfacesare so selected that the refractive index ivy/N,- is constant at eachsurface, Formula 1 is simplified to The sign convention for (1) and (2)is as follows:

C and n have the same sign if Vj+1 Vj and-opposite signs if V;i+1 V7'.

It is thus seen that by measuring the axial radius of curvature of eachequipotential the image curvature on the axis is easily found, and henceby employing said formula the design of lens according to the inventionis facilitated.

I claim:

1. A circuit arrangement comprising an electron discharge device havingan electrostatic electron lens system for focussing a composite beam ofelectrons, a source of electrons for producing a composite electronbeam, a plurality of electrodes, means for applying electric potentialsto a portion of said electrodes to produce an electron lens forconverging pencils of electrons from said source towards the axis of thesystem, means for applying electric potentials to a portion of saidelectrodes to produce a second electron lens for focussing saidconvergent pencils of electrons, said latter means for applying electricpotentials being proportioned to maintain an electrode of said secondlens which is more remote from said source at a lower potential thanthat of an electrode of said second lens nearer the source.

2. A circuit arrangement according to claim 1, said source being a planephotoelectric cathode.

3. A circuit arrangement comprising an electron discharge device havingan electrostatic electron lens system for focussing a composite beam ofelectrons, a source of electrons for producing a composite electronbeam, a plurality of electrodes, means for applying electric potentialsto a portion of said electrodes to produce an electron lens forconverging pencils of electrons from said source towards the axis of thesystem, means for applying electric potentials to a portion of saidelectrodes to produce a second electron lens for focussing saidconvergent pencils of electrons, said latter means for applying electricpotentials being proportioned to maintain an electrode of said secondlens which is more remote from said source at a potential less than 0.4that of an electrode of said second lens nearer to said source.

i. A circuit arrangement according to claim 3, said source being a planephotoelectric cathode.

BERNARD JOSEPH MAYO.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,131,185 Knoll Sept. 2'7, 19382,176,974: McGee et al. Oct. 24, 1939 2,185,378 Maloff et al. Jan. 2,1940 2,189,321 Morton Feb. 6, 1940 2,219,117 Schade Oct. 22, 19402,264,709 Nicoll Dec. 2, 1941 2,320,582 Flechsig June 1, 1943

